Smaller, Cheaper Biofuel Reactors

Researchers at the University of Minnesota have developed a fast way to convert sawdust and waste biomass directly into a mixture of gases that can be burned to generate electricity or made into liquid fuels such as diesel. If the process can be scaled up, it could be a more energy-efficient method for making biofuels by allowing for small, fast reactors located close to biomass sources.

The researchers developed a system that makes it possible to transform solids directly into a useful mixture of gases. The process begins when millimeter-sized particles come into contact with a 700 to 800 degree Celsius porous surface and instantly form a mixture of gaseous compounds. These interact with a catalyst made of the precious metal rhodium that facilitates partial oxidation reactions that both keep the system hot and convert the gases to hydrogen and carbon monoxide. This mixture of gases, called syngas or synthesis gas, can then be burned in a gas turbine to make electricity, or purified and made into a number of different fuels using well-known processes.

The key to the new process is a catalyst bed with the right kind of porous structure to maintain the temperatures and movement of materials needed for the chemical reactions. The resulting system breaks down the biomass in just 70 milliseconds. That is ten times faster than other methods for making syngas, says Lanny Schmidt, professor of chemical engineering and materials science at the University of Minnesota. Ideally, that means a reactor with a given volume could make ten times the amount of syngas using the new method than it could using conventional methods. Or put another way, it could allow for reactors one-tenth the size, he says.

The catalytic approach is one of a number of methods in development that could convert cheap sources of cellulosic biomass, such as sawdust, grass, and agricultural waste, into liquid fuels. It’s still not clear which of two broad categories of approaches will be more practical, thermochemical methods, such as Schmidt’s, or methods that use enzymes and organisms. Thermochemical methods are expensive but have the potential advantage of being able to use a number of different source materials, whereas biological systems will likely need to be fine-tuned for particular feedstocks.

But the ability to make smaller reactors for converting waste biomass to syngas could help meet one of the most significant challenges of producing fuels from biomass. Transporting bulky materials such as wood chips and corn waste long distances to central facilities uses a lot of energy, often in the form of fossil fuels. It also makes the overall process more expensive. Small, distributed syngas plants could cut down on these transportation costs by decreasing the distance the biomass has to be shipped. Distributed reactors could also be valuable in developing economies, Schmidt says, providing power and fuel to communities that don’t have reliable transportation infrastructure.

The overall affordability of such a system will partly depend on whether rhodium, which can cost upwards of $6,000 an ounce, can be used in small enough amounts–and over long enough periods of time. The process also has to be scaled up, even for small distributed systems. Right now, the prototype uses an experimental catalyst bed the size of a person’s thumb. The researchers estimate that a system that can make enough syngas to produce 10 gallons of gasoline a day would require a catalyst bed many times this size, about 15 centimeters across and 3 deep. It could prove difficult, says Theodore Krause, head of basic and applied sciences at Argonne National Laboratory, to make a larger system that remains fast and efficient.

While challenges remain, Schmidt’s system represents a distinct advance in the science of making fuels from biomass, Krause says. In demonstrating the ability to convert solids directly into syngas, he adds, the research has “demonstrated something that most people would have at first guessed was not possible.”